The present invention relates to thermoforming and more particularly to a method and apparatus for twinsheet forming thermoplastics.
Thermoforming is widely used in a variety of industries to manufacture products from thermoplastic materials. A typical thermoforming process involves the general steps of heating a sheet of thermoplastic material above its glass-transition temperature, shaping the heated thermoplastic by drawing the material over a thermoforming tool, such as a vacuum mold, cooling the thermoplastic to a low enough temperature for it to retain shape and then removing the shaped thermoplastic from the mold.
In a twinsheet thermoforming process, two sheets of thermoplastic are individually formed and then fused together to complete a single article. According to conventional twinsheet forming methods, first and second sheets of thermoplastic material are heated and thermoformed using first and second thermoforming tools, such as vacuum molds, positioned opposite one another. Before the sheet surfaces facing away from the tools have cooled, the tools are moved together or closed so that the hot surfaces of the sheets fuse together proximate the edges or in any other area that protrudes and contacts the opposing sheet. The residual heat and soft surface of the thermoplastic materials allows the sheets to fuse together.
One type of conventional twin sheet thermoforming system 10, FIG. 1, uses a carousel machine to form two thermoplastic sheets and fuse them together. The carousel machine has four stations 20, 22, 24, 26 and uses four clamp frames to rotate and index the sheets 90 degrees to each of the stations. The sheets are loaded sequentially at a load/unload station 20, for example, by clamping the first sheet with a clamp frame, indexing the carousel, and then clamping the second sheet with a clamp frame. The clamp frames are then sequentially indexed through first and second heating stations 22, 24. For example, the first sheet is heated in the first heating station 22 while the second sheet is loaded. The carousel then rotates 90 degrees to move the first sheet to the second heating station 24 and the second sheet to the first heating station 22. The two sheets are then simultaneously heated in the separate heating stations.
After being appropriately heated, the carousel is again indexed forward 90 degrees to move the first sheet to the forming station 26 and the second sheet to the second heating station 24. The forming station 26, includes upper and lower platens that support opposing upper and lower vacuum molds. After entering the forming station, the first sheet is formed in one vacuum mold. While the first sheet is being formed, the second sheet is maintained at the appropriate temperature at the second heating station 24. Once the first sheet is formed, it is released from the clamp frame. The carousel is then indexed forward 90 degrees to move the second sheet to the forming station 26. Because it has been released from the clamp frame, the first sheet remains on the mold. The second sheet is then formed in the other vacuum mold. The upper and lower platens are then driven together, causing the two thermoformed sheets to come into contact under pressure at various locations. The causes the two sheets to fuse together to form a completed part. The complete part is then indexed to the load/unload station 20 for unloading.
Problems arise in conventional twinsheet arrangements when fusing the first and second sheets together. In order to obtain proper fusion between two thermoplastic sheets, each sheet must be properly formed and then maintained at a high enough temperature that its material properties allow it to be fused with another sheet. After a sheet is heated and formed, there is only a small window of time in which the material is still capable of being fused. Beyond this time, the material has cooled to a point that it has become too hard to be fused with another thermoplastic.
The conventional four-station carousel system presents an obstacle to proper fusion in that it requires the second sheet to be heated and formed at a time later than the first sheet. As a result, the first sheet begins to cool while the second sheet is still forming, causing a temperature difference between the two sheets and further shrinking the window of time for proper fusion. A weak bond between the first and second sheets often results from the sequential timing of the conventional system, and in a worst case scenario the first sheet has cooled so much during the forming of the second sheet that no amount of fusion is possible. Certain thermoplastic materials, such as ABS and acrylic, have a very tight range of temperatures in which they are capable of being thermoformed and fused, and are incapable of being properly formed in the conventional twinsheet process because the sequential system cannot accommodate their short cooling time window.
Accordingly, a need exists for a twinsheet forming system capable of coordinating the heating, forming, and cooling times of the first and second thermoplastic sheets such that both sheets are formed and at a proper temperature for fusion at the same time in the process.